A suitable process for the preparation of storage stable poly(meth)acrylate secondary dispersions for binders or binder components in water-dilutable coating compositions has been described in EP-A 0 841 352. In that specification a mixture of ethylenically unsaturated monomers is polymerized in a mixture of organic solvents, initiator and water. The polyacrylate resin obtained is then at least partly neutralized and dispersed in water.
A disadvantage of the process described in EP-A 0 841 352 is that the resulting aqueous/organic dispersions are not sufficiently heat stable. The dispersions have inadequate heat stability because they are unstable if they are stored at elevated temperatures of 40° C. and above. This is apparent in particular if non-polar, poorly water-soluble cosolvents, e.g., a mixture of alkylated aromatics (such as Solvesso 100 solvent) are employed or the cosolvent/water ratio initially employed in the feed polymerization is kept below a weight ratio of 40/60.
Other preparation processes for secondary dispersions include so-called “two-pot processes”, wherein the polymerization is carried out in a homogeneous organic medium in a first tank and the resulting product is dispersed in an aqueous phase with prior or simultaneous neutralization, e.g. by means of amines, in a second tank. Such processes are described, for example, in EP-A 358 979, DE-A 198 14 061 and EP-A 1 024 184. These two-pot processes are expensive in technical terms because two autoclaves are required for the preparation of the dispersions and the transfer of the usually highly viscous polymer solution into the dispersing tank presents complications. Furthermore, in the initial stage of the polymerization there may also be problems with temperature regulation and stirring of the batch because of the small amount of solvent in the initial polymerization mixture due to the process technology. The addition of another dispersing medium, in general water, with the organic solvent is therefore necessary.
An object of the present invention is to provide heat stable vinyl polymer secondary dispersions which do not have the disadvantages described by the prior art and which are prepared by a process which is less expensive in technical terms. The storage stable secondary dispersions should also be suitable as coating compositions for practically any desired substrates, but in particular for metallic substrates.
These objects may be achieved with the polymer dispersions of the present invention. It has now been found that an adequate heat stability can be achieved in vinyl polymer secondary dispersions by the presence of suitable amphiphilic oligomers or polymers with a block or graft structure. The polymerization, which proceeds as a multiphase reaction in the cosolvent/water mixture and in the presence of the amphiphilic compounds, results in polyol dispersions that are storage stable even at temperatures above 40° C. The amphiphilic compounds act as stabilizing surfactants both during the polymerization and during the storage of the completely polymerized dispersions.